1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method that uses a print head for ejecting ink droplets to a print medium to form an image.
2. Description of the Related Art
An ink jet printing apparatus includes a print head in which a great number of print elements for discharging ink based on print information are arranged. In a serial-type ink jet printing apparatus, a main scanning operation for scanning a carriage mounting a print head in parallel with the flat surface of a print medium for printing and a transportation for transporting the print medium in a direction crossing the main scanning are alternately performed to form an image.
Some structures for ejecting ink from a print element have been already suggested and carried out. Among them, a structure in which a print element includes an electrothermal conversion element (heater) has been widely used because this structure can eject small ink droplets with a high density and a high frequency. In such an ink jet print head, each of print elements includes a liquid path for introducing ink to a ejecting opening and an electrothermal conversion element (heater) having a contact with ink in the liquid path.
When ink is ejected from a print element depending on an image signal, each heater is applied with a predetermined voltage pulse to heat a heater to heat ink. When being suddenly heated, film boiling is caused in ink having a contact with the surface of the heater to cause bubbles. The bubbles grow to push ink out. The ejected ink droplets fly to reach the print medium, thereby forming dots.
An ejection volume is directly influenced by a temperature of ink in the vicinity of the heater. For example, a print head having a low temperature causes a smaller volume of bubbles, a smaller ejection volume, and a smaller area of printed dots. A print head having a high temperature on the contrary causes a larger volume of bubbles, a larger ejection volume, and a larger area of printed dots. Specifically, a print head having an unstable temperature causes, even when identical image data is subjected to a printing operation, variation in the size of dots formed on a print medium and thus in the image density, causing a risk of uneven density.
A print head including a heater cannot structurally avoid fluctuation or variation in the ink temperature due to an environment in which the apparatus is used or the frequency of use of the respective color heads. However, variation in an image density in an ink jet printing apparatus due to reasons other than data is not preferred from the viewpoint of quality. Thus, stabilization of an ejection volume of a print head has been a major objective of an ink jet printing apparatus.
Japanese Patent No. 3247412 discloses a technique by which a voltage pulse is applied two times for one ink ejecting operation and a pulse width is controlled in a stepwise manner depending on the temperature of a print head to stabilize an ejection volume. Hereinafter, such a control of an ejection volume will be referred to as a PWM (Pulse Width Modulation) driving control.
FIG. 1A is a timing chart for explaining the PWM driving control. In FIG. 1A, the horizontal axis represents time and the vertical axis represents a value of a voltage applied to a heater. The two pulses shown in FIG. 1A are used to perform one ink ejection. In FIG. 1A, “P1” denotes a time during which a preheat pulse is applied, “P3” denotes a time during which a main heat pulse is applied, and “P2” denotes an interval between a preheat pulse and a main heat pulse.
A preheat pulse is a pulse applied to heat ink in the vicinity of the surface of a heater and is applied for the application time P1 so as not to cause energy causing foaming. The interval is determined so that mutual interference between a preheat pulse and a main heat pulse is prevented, thermal energy obtained by a preheat pulse is dispersed in ink, and a preferred temperature distribution is obtained. A main heat pulse on the other hand is a pulse applied to cause ink heated by a preheat pulse to have film boiling to perform ink ejection and is applied for the application time P3 longer than the application time P1 so as to provide sufficient energy causing foaming. A main heat pulse has a pulse width P3 determined based on a heater area, a resistance value, a film structure, or the structure of a liquid path.
As described above, an ink ejection volume depends on the temperature distribution of ink in the vicinity of a heater. Japanese Patent No. 3247412 adjusts, depending on a detected temperature, a pulse width P1 of a preheat pulse or an interval time P2 (input energy and input time) to control the ink temperature distribution (i.e., foaming region) to control an ink ejection volume. This will be described specifically. When a detected temperature gradually increases for example, the necessity for heating ink at the surface of a heater is gradually reduced. In this case, the preheat pulse width P1 is gradually reduced. When a detected temperature gradually decreases on the other hand, the necessity for heating ink at the surface of a heater is increased and thus the preheat pulse width P1 is gradually increased.
When heat is more accumulated at the print head to reach the preheat pulse P1 of 0, only a main heat pulse as shown in FIG. 1B is used as a driving pulse and further PWM driving control is impossible. Specifically, the PWM driving control disclosed in Japanese Patent No. 3247412 can control an ink ejection volume in a temperature range until the preheat pulse P1 is 0.
Japanese Patent Laid-Open No. 2001-180015 on the other hand discloses a method to stabilize an ink ejection volume by simultaneously changing an applied voltage value and a pulse width.
FIG. 2 is a timing chart for explaining the driving control method disclosed in Japanese Patent Laid-Open No. 2001-180015. An ink jet print head having a heater is characterized in that a pulse 302 for applying a high voltage Vop2 to a heater for a short time causes a smaller ink ejection volume than that caused by a pulse 301 for applying a low voltage Vop1 to a heater for a longer time. The reason will be described hereinafter.
When a voltage pulse is applied to a heater, ink in the vicinity of the boundary surface of a heater is firstly heated to transmit heat to ink at the periphery. Further heating of the ink causes ink in the vicinity of the boundary surface to foam to eject ink in an amount corresponding to the volume of the foamed ink. The volume of foamed ink depends on the number of molecules of vaporized ink. This number of molecules of vaporized ink is determined based on the volume of ink receiving sufficient heat quantity from the heater until the ink foaming is caused. In this case, an increased voltage applied to an electrothermal conversion element starts the vaporization of ink near the boundary surface with a smaller range in which heat can be transmitted to ink. Since gas has a very small thermal conductivity, the heater after the ink foaming is substantially heat-insulative to substantially prevent heat from being newly transferred to liquid existing at the periphery. As a result, the volume of vaporized ink molecules of small number is a volume of foamed ink, thus causing a smaller amount of ink to be ejected.
Japanese Patent Laid-Open No. 2001-180015 uses the characteristic as described above for controlling an ink ejection volume. Specifically, when an increased ink ejection volume is desired, a pulse shape is determined so that a driving voltage is reduced and a pulse width is increased. When a reduced ink ejection volume is desired, a pulse shape is determined so that a driving voltage is increased and a pulse width is reduced. Although the above description has been made with a single pulse for simplicity, such a characteristic also can be checked by using a double pulse.
Japanese Patent No. 3158381 discloses a technique that combines the general PWM control disclosed in Japanese Patent No. 3247412 with the technique disclosed in Japanese Patent Laid-Open No. 2001-180015 so that a PWM driving control can be performed in a further broader temperature range. According to the general PWM driving control as disclosed in Japanese Patent No. 3247412, the driving voltage Vop is retained constant in all controllable temperature regions. At a temperature higher than that of a condition in which a preheat pulse width is 0 as shown in FIG. 1B, the control of an ink ejection volume is impossible. However, even in such as high temperature region, an ejection volume control width can be further expanded to a high temperature range by resetting a driving voltage at a further higher value.
FIG. 3 shows an example of a pulse table when the two driving voltages vop1 and voP2 are used to perform the PWM driving control. A print head includes a temperature sensor for detecting the temperature of ink near a heater. A detected temperature is compared with a threshold temperature value to select a corresponding pulse. When detected temperature show a relation of T1<T<T2 for example, a pulse of tb15 is set. When detected temperature show a relation of T1<T<T6, five pulses of tb15 to tb11 correspond, these driving voltages are fixed to Vop1. Specifically, in a temperature region of T1<T<T6, the PWM control based on the driving voltage Vop1 is performed.
When detected temperature show a relation of T6<T<T11 on the other hand, five pulses of tb25 to tb21 are prepared, these driving voltages are fixed to Vop2 higher than Vop1. Specifically, in a temperature region of T6<T<T11, the PWM control based on the driving voltage Vop2 is performed. FIG. 4 illustrates the PWM control explained above. In FIG. 4, as the detected temperature T is higher, an applicable table is selected in order. A control range TW includes two types of PWM control regions, which are PWM control region for 20.5V and PWM control region for 24.0V. It is understandable that the two types of PWM control are switched on reaching 60 degrees. When the driving control is performed in such a high temperature region based on a high driving voltage, the heater is applied with a lower energy than in the case of the driving with a lower voltage. Thus, further accumulation of heat at the print head can be more actively suppressed.
As described above, the PWM driving control method in two steps can be performed in a continuous manner to control an ejection volume of a print head in a broader temperature range (T1 to T11).
By the way, as described above for the serial-type ink jet printing apparatus, when a relatively small printing apparatus for consumers for example is subjected to a driving control, the detection of a temperature of a print head and the switching of a driving pulse may be performed at every timing of main scanning for a print operation is performed. The reason is that, in such a printing apparatus, temperature fluctuation during one print scanning is not so high and density fluctuation can be suppressed to prevent a problematic image quality so long as a pulse is reconsidered and switched whenever a print scanning is performed.
In the case of a large ink jet printing apparatus for business use on the other hand, a print scanning is performed over a longer distance to cause a proportionally higher temperature fluctuation during a print scanning. Specifically, such density fluctuation may be caused during a print scanning that causes a problematic image quality. Thus, such a large ink jet printing apparatus is desirably structured so that a temperature of a print head is detected and a driving pulse is switched even during each print scanning.
However, although a pulse width can be changed during a print scanning, a driving voltage is difficultly changed. For example, with reference to FIG. 3, when a detected temperature fluctuates from T5 to T6 during a print scanning, two different voltages of Vop1 and Vop2 are demanded to be supplied in a single print scanning. In order to realize this, a more complicated and larger power source supply circuit is required, thus causing a significant increase of cost.